1. Field of the Invention
This invention relates to heating circuits, particularly (but not exclusively) for electric blankets or pads.
2. Description of the Prior Art
It is known to incorporate in an electric blanket a cable comprising a heating conductor, a sensor conductor and separating means that separates the two conductors and which has an impedance that falls with increasing temperature and/or which will melt in the event of overheating to allow contact between the two conductors. (See U.S. Pat. No. 2,831,099 to Crowley and UK Pat. No. 1,155,118 issued to the assignees hereof.) An AC power supply is connected across the heating conductor to heat it. Means is provided responsive to the impedance of the separating means dropping to a value indicating overheating of the cable (which value will be zero in the event of melting resulting in contact of the conductors) to prevent current flowing through the heating conductor. A disadvantage of such an arrangement is that the reliable detection of overheating becomes difficult due to the voltage gradient occurring along the heating conductor due to the fact that the supply voltage is applied across its ends. While this does not create a problem if the overheating is general, i.e., if it is present along the whole length of the cable, a difficulty arises if the overheating is localised at a position along the length of the cable, because the voltage at the position where the two conductors are connected by the reduced or zero impedance will be anywhere between the full supply voltage and zero, depending on the location of the position.
Known attempts to overcome this disadvantage--see the above-mentioned U.S. Pat. No. 2,831,099 and also UK Pat. No. 793,783--have involved the use of complex and thus unreliable circuit arrangements.
A primary object of the present invention is to overcome the above-mentioned disadvantage of the prior art.
Another object of the invention is to overcome the above-mentioned disadvantage of the prior art by enabling the reliable detection of both localised and general overheating in a simple manner, without the need for complex circuit arrangements and/or an additional sensor conductor.
A further object of the invention is to reliably detect overheating resulting in short-circuiting between a cable incorporating the two conductors in a manner which will avoid disintegration of the cable at the location of the short circuit.
Yet another object of the invention is to provide a heating circuit which reliably detects overheating, both localised and general, which is of simple construction, and which provides a selective multiheat output, preferably with two heating conductors only.